1. Field of the Invention
This invention relates to the field of chemistry, and more specifically to the conversion of a methane-containing gas-phase input stream to a C5-C19 carbon containing liquid-phase output stream that is usable as a compression ignition fuel (i.e., as diesel fuel) without the need for further processing.
2. Description of the Related Art
This invention utilizes at least one Catalytic Partial Oxidation (CPOX) reactor.
Known technologies for converting natural gas, which is mostly methane, into a synthesis gas (syngas) that is a mixture of hydrogen and carbon monoxide, include the CPOX reaction. The CPOX reaction is a mildly exothermic process. CPOX of methane produces a syngas stream having a hydrogen-to-carbon monoxide ratio of about 2, this being close to the optimum that is required by a Fischer-Tropsch reaction.
U.S. Pat. No. 5,883,138, incorporated herein by reference, describes a reactor apparatus for the partial oxidation of light hydrocarbon gases, such as methane, to convert such gases to synthesis gas for recovery and/or subsequent hydrocarbon synthesis.
The present invention also utilizes at least one Fischer-Tropsch reactor.
The Fischer-Tropsch reaction is a well-known mechanism for hydrogenating carbon monoxide or synthesis gas into a mixture of olefins, paraffins, and oxygenates in the presence of transition metal based catalysts. Such catalysts may incorporate a first row non-noble metal such as iron, cobalt, or nickel as the predominant active site, along with a noble metal (ruthenium, platinum, rhenium), actinide (thorium), or alkali (lithium, sodium, potassium) promoter, optionally supported on a refractory, non-reducible, oxide such as silica, alumina, or titania.
Conversion of synthesis gas by way of the Fischer-Tropsch reaction occurs as a result of the following highly-exothermic chemical process.
CO+2H2xe2x86x92CH2+H2O
A cobalt-based Fischer-Tropsch catalyst is a catalyst of choice for the conversion of synthesis gas to-liquid fuels due to the high activity and the long life of this type of catalyst. A tubular-fixed bed Fischer-Tropsch reactor or a slurry-phase Fischer-Tropsch reactor can be used, with temperature control being less of a problem when a slurry-phase Fischer-Tropsch reactor is used.
Wax and hydrocarbon condensate that is produced by the slurry-phase Fischer-Tropsch process are predominantly linear paraffin wax having a small fraction of olefin and oxygenate. Hydrogenation of the olefins and oxygenates, and hydro-cracking of the wax to naphtha and diesel can be done under relatively mild conditions.
It is known that gas-phase hydrocarbons can be converted into liquid-phase hydrocarbons via a two step process such as is shown in U.S. Pat. No. 5,620,670 to Benham et al., of which U.S. Pat. No. 5,324,335 is a division, both incorporated herein by reference.
U.S. Pat. No. 5,620,670 teaches converting a hydrocarbon-containing gas into liquid hydrocarbon products that have a carbon content between C5 and C20. In this patent, a first reaction converts a hydrocarbon-containing or methane-rich feed into hydrogen and carbon monoxide in the presence of carbon dioxide. The hydrogen and carbon monoxide are then reacted in a Fischer-Tropsch reactor using a promoted iron oxide or iron-based unsupported catalyst, to thereby form liquid hydrocarbon products, including diesel fuels. Partial oxidation (POX) and steam reforming can be used to convert the hydrogen-containing gases into a mixture of hydrogen and carbon monoxide. That is, POX and steam reforming can be used to produce synthesis gas from methane. In both of these processes, high temperatures and low pressures are said to favor production of the synthesis gas, with POX being favored because it is self-sustaining; i.e., it does not require the addition of heat once the reactants have been preheated. This patent states that two catalyst types that attract the most attention for the Fischer-Tropsch reactor are cobalt-based catalysts and iron-based catalysts, where cobalt-based catalysts approach 100% carbon conversion efficiency, whereas iron-based catalysts tend toward 50% carbon conversion efficiency during the Fischer-Tropsch synthesis reaction. It is suggested that iron-based catalyst used in the Fischer-Tropsch reactor be a precipitated iron catalyst, and most preferably, an unsupported precipitated iron catalyst that is promoted with predetermined amounts of potassium and copper using elemental iron and copper as starting materials.
Also of interest is U.S. Pat. No. 6,169,120 to Beer, incorporated herein by reference. This patent describes a two-stage, slurry bubble column, Fischer-Tropsch synthesis process that is particularly adapted for use with synthesis gas containing nitrogen. Two Fischer-Tropsch reactors each contain a catalyst comprising cobalt, ruthenium, or cobalt and ruthenium on a support comprising at least one inorganic metal oxide selected from Group IIIA, IIIB, IVB, VB, VIB and VIIB metal oxides, alumina, silica, silica alumina, and combinations thereof, at a temperature from about 380 to about 500 degrees F., at a pressure from about 15 to about 25 atmospheres, and at a carbon monoxide conversion from about 40 to about 60 percent, to produce a liquid hydrocarbon product. A separator that is located downstream from the second reactor provides a C5 -C17 hydrocarbon output stream.
U.S. Pat. No. 4,568,663 to Mauldin, incorporated herein by reference, states that natural gas, or methane, can be converted into synthesis gas, that conversion of the synthesis gas to hydrocarbons can be carried out via Fischer-Tropsch synthesis, and that the use of Fischer-Tropsch synthesis for the production of hydrocarbons from carbon monoxide and hydrogen are well known. It is also stated that promoted and supported Group VIII non-noble metals iron, cobalt and nickel have been used in Fischer-Tropsch reactions. This patent provides a supported cobalt catalyst, notably cobalt titania (cobalt titanium dioxide) and cobalt thoria titania (cobalt thorium dioxide titanium dioxide) for use in methanol conversion reactions in Fischer-Tropsch synthesis. A particulate catalyst is described consisting of a catalytically active amount of cobalt, or cobalt and thoria, to which rhenium is added.
The present invention provides a method and an apparatus for the production of high molecular weight fuel-grade liquid hydrocarbons from gas phase low molecular weight hydrocarbons. That is, fuel-grade liquid hydrocarbons are produced, the fuel-grade liquid hydrocarbons having a larger number of carbon atoms per molecule than do the gas-phase hydrocarbons.
In accordance with the invention, a mixture of air (about 79 percent by weight nitrogen and about 21 percent by weight oxygen) and gas phase, low molecular weight hydrocarbons (for example, natural gas or methane, CH4) is processed by a series reactor arrangement having an initial CPOX reactor followed by one or more Fischer-Tropsch reactors, wherein each of the one or more Fischer-Tropsch reactors provides a high molecular weight, fuel-grade, liquid-phase hydrocarbon output, for example, a liquid hydrocarbon output having a carbon content of from about C5 to about C19.
This fuel-grade liquid hydrocarbon output finds utility as a fuel for compression ignition internal combustion engines; i.e., diesel engines, without the need for hydro-processing.
The invention is comprised of at least three-step reaction process steps.
The first reaction step utilizes a CPOX reactor wherein a gas-phase hydrocarbon input is passed in contact with a first catalyst to produce a first gaseous mixture of carbon monoxide and hydrogen.
The first catalyst is a platinum group catalyst, a promoted platinum group catalyst, a rhodium catalyst, or a platinum promoted rhodium catalyst.
The second reaction step utilizes a synthesis zone (i.e. a first Fischer-Tropsch reactor) wherein the above-mentioned first gaseous mixture of carbon monoxide and hydrogen is passed in contact with a second catalyst, and is thus converted into a mixture of high molecular weight liquid-phase hydrocarbons and low molecular weight gas-phase hydrocarbons.
The second catalyst is made up of from about 3 to about 60 parts by weight cobalt and from about 0.1 to about 100 parts by weight of at least one metal selected from a group consisting of cerium, lanthanum, platinum, and ruthenium per 100 parts by weight of a support selected from a group consisting of silica, alumina, and a combinations of silica and alumina. A preferred second catalyst is made up of about 20 percent by weight cobalt, about 0.1 percent by weight ruthenium, about 1.0 percent by weight platinum, the remainder being alumina support.
The liquid-phase output of this second reaction step comprises a soft wax and naphtha fractions simultaneously with a middle distillate carbon constituent that boils in the traditional diesel temperature range, this being the formulation of the output of the second reaction step (i.e., the output of the first Fischer-Tropsch reactor).
This liquid-phase output of the first Fischer-Tropsch reactor finds direct utility, without further processing, as a middle distillate, compression ignition, fuel exhibiting about 30 percent naphtha by weigh.
The retention of both naphtha and soft wax within the liquid-phase output of the first Fischer-Tropsch reactor (as opposed to the prior art use of hydro-processing) adds value to the diesel fuel by way of the diesel fuel naphtha fraction, and permits direct utilization of the output of the first Fischer-Tropsch reactor as a compression-ignition fuel.
Savings on the order of from about 10 to about 15 percent are realized as a result of the elimination of the need to hydro-process the output of this first Fischer-Tropsch reactor.
Direct production of a lubricity additive, capable of providing polar functionalities such as hydroxyl or carbonyl groups, is an extension of the output of the first Fischer-Tropsch reactor. In particular, higher alcohols of carbon number 12 to 18 can be produced in-situ in a fashion analogous to the production of naphtha, middle distillate and soft wax via the first Fischer-Tropsch reaction of the invention.
As a third step feature of the invention, a low molecular weight gas-phase output of the first Fischer-Tropsch reactor may be applied to a second Fischer-Tropsch reactor, to thereby produce another high molecular weight liquid-phase output from the second Fischer-Tropsch reactor. This liquid-phase output also finds direct utility, without further processing as a middle distillate compression-ignition fuel.